According to Webster's New International Dictionary-Unabridged, 1957, page 1748, "OZONE is a faintly blue, gaseous, allotropic form of oxygen, present in minute amounts in the atmosphere, especially in the upper regions, and obtained (usually much diluted) by the silent discharge of electricity in ordinary oxygen or in air, and by other methods; it has the molecular formula 0.sub.3, being one and one half times as dense as ordinary oxygen gas. It is a much more powerful oxidizer than oxygen gas, and comparatively unstable, (i.e.: decomposes at high temperatures). Ozone is used commercially for sterilizing water, purifying air, bleaching, etc."
In the known prior art "silent discharge" devices ozone is produced by passing oxygen or air through long discharge tubes made of concentric glass cylinders which are metal-coated on innermost and outermost walls, the gas being passed through the annular space between the cylinders. Low-frequency, high voltage alternating current is applied to the metal coatings and ozone is formed in the gas. An illustration of such an ozonizer can be found in FIG. 2 on page 1264 of Van Nostrand's Scientific Encyclopedia fourth Edition, published by D. Van Nostrand Company, Inc., Princeton, N.J., 1968 (Library of Congress Catalog Card No. 68-20922). This process of producing ozone by "silent discharge" has been found to be inefficient. The alternating current discharge produces positive and negative ions together with the ozone and, among other things these oppositely charged ions can recombine, generating heat in the gas. Ozone is destroyed by high temperatures and the recombinations of oppositely-charged ions may destroy some of the ozone produced in such devices and does in fact destroy ozone in many prior art devices. Cooling of the ozonizer walls is required to prevent thermal decomposition of the ozone produced. Moreover, moisture greatly reduces the efficiency of ozone production with the silent discharge. In consequence, according to the known current practice, air (or other oxygen-bearing gas) that is to be used for ozone production is refrigerated and dried to a dewpoint of minus 60.degree. F before it is introduced into the "silent discharge" ozone generator. This is costly and the required equipment occupies a great deal of space, and consumes energy.
It now appears to be generally accepted that ozone will replace chlorine for most types of water treatment provided that the costs involved can be adequately decreased. At present, using silent discharge, 10 to 15 kilowatt-hours of electric power are required to generate one pound of ozone. Part of this power is consumed by the air or oxygen pretreatment equipment. Chlorine requires less than half as much energy, resulting in substantially lower costs. The present invention makes use of the fact that ozone can be produced more efficiently by a negative electric discharge, for example, an electron beam, than by an alternating current silent discharge. When an electron beam is employed in practicing the invention, moisture in the gas to be treated by an electron beam will not materially degrade the efficiency of ozone generation; water molecules will be split into H, H, and O -- and the O atoms will combines with other O atoms to form oxygen and ozone. In accordance with the invention, the air or other oxygen-bearing gas which is treated to produce ozone is recirculated in a closed loop, to be irradiated again and again by one or more negative electric discharge ozone generators. Any ozone that does not decompose in the recirculation path will recirculate and will have added to it ozone newly produced on each pass of the gas by the generator(s). Any ozone disrupted by negative ions can recombine again as either ozone or oxygen. Concentration of ozone in an oxygen-bearing recirculating gas will thus rise until an equilibrium is reached between ozone-bearing gas leaving the system and oxygen-bearing gas that enters it. In the present invention employing a negative electric discharge it is possible to reach an ozone concentration well in excess of 10,000 parts per million in air or in a similar recirculating carrier gas. The invention thus contributes new and more useful methods and means to concentrate ozone.
When ozone is produced for water treatment, it is desirable to have about 1 to 3 parts per million ozone dissolved in the water which is being treated, a greater amount being required where other ozone traps or ozone-consuming products are in the water. Thus, relatively good, clean water can be purified or disinfected with about 1 p.p.m.; typical sewage containing organic waste, soaps and detergents, industrial waste and the like is generally better treated with 3 p.p.m. dissolved ozone. According to Henry's Law, the concentration of ozone that can be introduced into water by mixing ozone-bearing air and water in equilibrium conditions varies with the concentration (or partial pressure) of ozone in the air, and to produce about 3 p.p.m. ozone in the water a concentration of 10,000 p.p.m. (or about 1%) ozone in air is a valuable threshold objective.
In other words, if ozone generation is to be a useful art in water purification, it should produce at least about 10,000 p.p.m. ozone in air or other carrier gas, should have a production efficiency about as good as or preferably better than about 6 or 7 k.w.h. per pound, and should not require expensive or dangerous processing steps. Specifically, it should be possible to use air rather than pure oxygen as its raw material and should not require the expenditure of additional energy to pretreat or dry the air.